They need to train their ushers to be more aggressive with people like that. There needs to be people reminding patrons, huge signs, and several announcements. They need to remove people from the theater whose cell phones go off. There is no excuse. If you're expecting a call, don't attend the show that day.
Unfortunately, a lot of people turn their phones back on during intermission and then forget to turn them off for act 2. I don't know that I've ever heard a reminder at intermission to turn the phone off again, and sadly a lot of people forget to turn 'em back off. The fact that screens are getting bigger again sometimes makes it easier to find them, but if the person is in the middle of a row, it is difficult to get their attention to tell them to turn it off. (And I have waved someone down in the middle of a row to tell them to turn it off.)